Display devices which employ a liquid crystal are widely used for watches, tabletop calculators, and others. These liquid crystal display devices employ optical anisotropy and dielectic anisotropy of a liquid crystal substance. Liquid crystal phase includes a nematic liquid crystal phase, smectic liquid crystal phase, and cholesteric liquid crystal phase. Devices which use a nematic liquid crystal phase are most widely put in practical use. Further, the mode used in liquid crystal display can be classified into TN (twisted nematic) type, DS (dynamic scattering) type, guest-host type, and DAP type from the aspect of electro-optic effect.
Heretofore, a large number of liquid crystalline compounds including those at a research stage are widely known. At present, however, no compound has been found which is enclosed and used in a display device as a single liquid crystal compound. This is because whereas a liquid crystalline compound which is expected as a material for a display device is desired to exhibit a liquid crystal phase at a temperature range as wide as possible, centered at a particular temperature at which the liquid crystal substance is most frequently used, and whereas the liquid crystal substance must be sufficiently stable against environmental factors to be used and must have physical properties sufficient to drive display devices, no substance which satisfies these requirements as a single compound has been found.
Then, several kinds of liquid crystalline compounds, that is, liquid crystal compound and non-liquid crystalline compounds are mixed to prepare compositions having such properties, and put in practical use as raw materials at present. These liquid crystal compositions are required to be stable even against moisture, light, heat, and air which usually exist under environment under which the compositions are used, and also they are required to be stable against electric field and electromagnetic radiation. Further, liquid crystalline compounds to be mixed are required to be stable each other under environment to be used. Besides, liquid crystal compositions are required to have appropriate values of such several physical properties as optical anisotropy, dielectric anisotropy, and electric conductivity depending on the modes of display and shape of devices. Particularly, the compositions are required such that the product of a value of their optical anisotropy (.DELTA.n) and thickness of a cell (d) show a constant value. Since a tendency for (d) mentioned above to reduce is increased to obtain good quality of displays having no domains in recent display devices, it has become important to increase the .DELTA.n of the compositions, and thus the need for a single liquid crystalline compound having a high .DELTA.n has become significant.
Heretofore, for examples, tolan derivatives expressed by the formula (a) below (Unexamined Japanese Patent Publication No. Hyo Hei 01-502823) or butadiyne derivatives expressed by the formula (b) below (Mol. Cryst. Liq. Cryst. 48, 175 (1978) are known in public. However, they have such defects that the former are narrow in their mesomorphic range and the latter are thermally instable. Thus neither material can be said to be sufficient to achieve the objects mentioned above.
Further, known stilbene derivatives expressed by the following formula (c) (Flussige Kristalle in Tabellen. Leipzig. VEB Deutsher Verlaugfur Grundstoffindustrie 1975, 49) and difluorostilbene derivatives expressed by the following formula (d) (Unexamined Japanese Patent Publication No. Hei 03-294386) which were recently developed with a purpose of stabilizing the stilbene derivatives are useful as raw materials for liquid crystal having a comparatively high .DELTA.n and a low viscosity. However, since neither material has a sufficiently wide mesomorphic range, they had no choice but to mix with other raw materials for liquid crystal having a high clearing point to compensate for the defects.
The present inventors have developed compounds having a conjugated eneyne structure as expressed by the following formula (e) with an object to develop new compounds which offset the defects of those materials mentioned above (Unexamined Japanese Patent Publication No. Hei 06-312946). However, even those compounds can hardly be said to have a sufficiently large .DELTA.n. ##STR2##